1. Field of the Invention
The present invention relates to a battery protection circuit device for preventing over-charge and over-discharge of a battery.
2. Description of the Related Art
A secondary battery, such as a lithium ion secondary battery, requires a protection circuit device for protecting the battery from becoming over-discharged or over-charged. Since in a battery having a plurality of secondary batteries connected in series the detection of an overall voltage alone poses a risk that a certain secondary battery may become over-charged or over-discharged due to a variation in the characteristics of the respective secondary batteries, each voltage of the secondary batteries is detected, and even only one of the secondary batteries that constitute the battery is protected from becoming over-discharged or over-charged (for example, refer to FIG. 3 of Japanese Patent 3,291,530).
A battery such as a lithium ion secondary battery having a plurality of secondary batteries connected in series produces a high voltage depending on the number of secondary batteries. For that reason, the withstanding voltage of elements constituting a protection circuit device for a battery needs to be raised in order not to be broken by an application of the high voltage. The sizes of a semiconductor element and an element separation region become larger as the withstanding voltage becomes higher.
Hence, in the protection circuit device for a battery having a plurality of secondary batteries connected in series, the size of the overall protection circuit device increases not only by the increase in the circuit size due to increase in the number of secondary batteries, but also by the increase in individual size of the semiconductor elements constituting the circuit.
A large number of patent applications related to a protection circuit for a battery having a plurality of secondary batteries connected in series have been filed up to now. However, there are few applications that explicitly refer to the withstanding voltage of the semiconductor element constituting the circuit. For example, in a circuit illustrated in FIG. 3 of Japanese Patent 3,291,530, there is no disclosure as to where the potential of a well is taken from, and how much the withstanding voltage of the element is. It is supposed from the drawings that because NOR gates G2 and G3 are the same, comparator circuits COMP1 to COMP4 are disposed between a ground and a power supply of an overall protection circuit. That is, an entire voltage of secondary batteries connected in series is potentially applied, and the withstanding voltage of the element is required for the amount as large as the entire voltage of the secondary batteries connected in series. In this example, because the withstanding voltage twice as large as that in the case of using one secondary battery is required as described above, the element size is increased so as to satisfy that withstanding voltage.
In this example, because two secondary batteries are used, the withstanding voltage is also doubled. When ten secondary batteries are connected in series, a tenfold withstanding voltage is required. That is, as the number of secondary batteries connected in series increases, the necessary withstanding voltage increases, and the element size increases. Accordingly, the protection circuit device for a battery becomes bigger.
Further, the characteristic of the semiconductor element is generally varied due to a process variation during manufacture. For that reason, a trimming circuit is normally incorporated into the semiconductor element. Because the variation becomes generally larger as the withstanding voltage is higher, a range to be trimmed is larger as the withstanding voltage is higher, the trimming circuit is upsized, and an area of the protection circuit device for a battery becomes larger.
Further, as the variation of the element characteristic is larger, the detection voltage when the temperature is changed is more deviated. This leads to the degradation of precision of the detection voltage. When the variation of the element characteristic is large as described above, the precision of various characteristics is degraded. The area of the protection circuit device increases when a circuit for correcting those characteristics is incorporated into the protection circuit device.
As described above, the area of the protection circuit device for the battery having a plurality of secondary batteries connected in series becomes larger. In the semiconductor element, an increase in area is directly linked to an increase in its cost. Hence, there arises such a problem that the protection circuit device for a battery cannot be inexpensively provided.